Plastic and reconstructive surgery often requires the use of graft materials for the replacement or augmentation of tissues. Materials used for this purpose heretofore have been of biologic or synthetic origin. Biologic materials of both autologous and homologous origin have been tried extensively. Both types of biologic material have been subject to unpredictable resorption, requiring the patient to undergo additional corrective surgery. The use of homologous implant materials, for example, collagen or bone, can also result in an adverse immunologic reaction that can lead to graft rejection and extrusion. While such adverse reactions do not occur with autologous implants, the use of autologous material involves additional surgical time and trauma for their removal.
Synthetic materials previously used for implantation have generally been polymeric, for example, silicone and polytetrafluoroethylene (hereinafter PTFE). Non-porous materials do not allow tissue ingrowth and as a consequence are known to migrate from the implant location. Preferred synthetic materials have a porous structure that promotes tissue ingrowth and stabilization of the implanted material.
Proplast.RTM., a carvable porous composite implant material comprising PTFE fibers, powdered PTFE resin and carbon or aluminum oxide, has been available for some time. This material and its methods of manufacture are described in U.S. Pat. Nos. 3,992,725 and 4,129,470. Briefly, this material is made by blending the above listed materials with a soluble filler, filtering the blend to produce a cake, pressing and heating the cake, drying the cake, sintering the cake, and finally leaching out the filler material and again drying the resulting porous composite. This implant material is carvable and allows tissue ingrowth. However, the use of carbon or aluminum oxide in this material increases its tissue reactivity, potentially resulting in undesirable complications such as encapsulation by fibrous tissue, erosion of overlying tissues and extrusion. Finally, the carbon impregnated material is often visible through the skin when implanted subcutaneously in light-skinned patients.
Pure PTFE, that is, PTFE without other added materials such as carbon, has a long history of use as an implantable material because it is one of the least reactive materials known. In porous form it can allow tissue ingrowth. Porous PTFE has been available for some time in a form known as expanded PTFE. The manufacture of this material is described in U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390. Expanded PTFE has a microstructure characterized by nodes interconnected by fibrils. This material has a history of use in such implant applications as vascular grafts, sutures and structural soft tissue repair including hernia repair and ligament augmentation and replacement. The porosity and microstructure of expanded PTFE can be varied to produce different permeability characteristics for use in a variety of applications.
Many implantable biodegradable synthetic polymers have been investigated and applied in various applications including the controlled time release of drugs and for medical devices such as sutures, prosthetic ligaments and bone repair. These polymers and their copolymers are chosen for specific applications according to their strength characteristics and their known rates of degradation. Their success in these applications is largely due to the following characteristics:
1) Adequate mechanical strength; PA0 2) Controlled rate of degradation; PA0 3) Complete absorbability without formation of toxic metabolytes; and PA0 4) Minimal inflammatory response from the host.
Frequently used implantable biodegradable synthetic homopolymers include polydioxanone (PDS), polyglycolic acid (PGA, also known as polyhydroxyacetic ester), polylactic acid (PLA) and polycaprolactone.
Copolymers of PGA/PLA are also commonly used. Copolymers of PGA/PLA typically degrade faster than either homopolymer PGA or PLA. Degradation rate is affected by the blend of the copolymer, the degree of crystallinity of the polymers, and the addition of other agents. The PGA degradation rate may also be sensitive to the rate of curing of the polymer, the fast-cured polymer appearing to degrade more quickly than the slow-cured.
In addition to synthetic polymeric materials, several biologically derived materials have been used for implantable biodegradable applications. Such biologically derived materials include albumin and collagen.